Dune and Other Desert Features

Photo by: Anton Prado PHOTO

Desert
environments have fascinated humans throughout the ages. Covering
approximately one-third of Earth's land surface, these arid (dry)
landscapes receive less than 10 inches (25 centimeters) of rain per year
and support only limited plant and animal life. Deserts may be hot,
located primarily between the tropic of Cancer and the tropic of
Capricorn, two parallel lines of latitude lying one-quarter of the way
from the equator to the North and South Poles, respectively. Deserts may
also be cold, located in polar regions where the mean temperature during
the warmest month is less than 50°F (10°C). Most deserts on
the planet lie within the Tropics (also called the Torrid Zone).

What differentiates deserts from other ecosystems (communities of plants
and animals interacting with their environment) around the world is not
only extreme climate but the landforms scattered across their surfaces.
For many people, these fantastic forms define the desert landscape. Miles
upon miles of rolling dunes, like waves on a sea suddenly stopped,
dominate popular images of deserts in books and motion pictures. Yet, only
one-fifth of all desert surfaces are covered with sand. Alluvial fans,
arroyos, blowouts, desert pavement, oases, playas, yardangs: These are but
a few of the many features that combine to create the spare desert
landscape.

The shape of the land

Although dunes, wind-blown piles of sand, make up only 20 percent of the
total desert landscape, these landforms may cover thousands of square
miles and reach heights of up to 1,640 feet (500 meters). Dunes occur in
many shapes, but common to all dunes is the contrast between the gentle
slope of the windward side (the side facing into the wind) and the steep
slope of the leeward side (the side facing away from the wind). The
leeward side is known as the slip face of the dune. Geologists,

Sand dunes in the Gobi Desert, the coldest and northernmost desert
in the world. The Gobi covers half a million square miles and is
located on a plateau that is 3,000 to 5,000 feet above sea level in
the heart of Asia.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

scientists who study the origin, history, and structure of Earth, have
classified the most common dune forms into five types: barchan, parabolic,
linear, transverse, and star.

Dune types

A barchan (pronounced bar-KAN) dune, sometimes known as a crescentic dune,
is a crescent or U-shaped dune that has its "horns" or tips
pointing downwind or away from the wind. Barchans arise where sand supply
is limited, where the ground is hard, and where wind direction is fairly
constant. They form around shrubs or larger rocks, which act as anchors to
hold the main part of the dune in place while the tips migrate with the
wind. Barchan dunes occur widely in deserts around Earth.

A parabolic dune is similar in shape to a barchan, but its tips point into
the wind. Its formation is also influenced by the presence of some type of
obstruction, such as a plant or a rock. Just the opposite of a barchan, a
parabolic is anchored at its tips by the obstruction, which acts to block
the wind, while its main body migrates with the wind, forming a depression
between the tips. Because of this formation, parabolic dunes are also
known as blowout dunes.

A linear, or longitudinal, dune is one that forms where sand is abundant
and cross winds converge, often along seacoasts where the winds from the
sea and winds from the land meet and push the sand into long lines. These
high, parallel dunes can be quite large: Scientists have recorded linear
dunes reaching 655 feet (200 meters) in height and 62 miles (103
kilometers) in length. The crests or summits of linear dunes are often
straight or slightly wavy.

A transverse dune also forms where sand supply is great. This dune is a
ridge of sand that forms perpendicular to the direction of the wind. The
slip face of a transverse dune is often very steep. A group of transverse
dunes resembles sand ripples on a large scale.

Dune and other desert features: Words to Know

Abrasion:

The erosion or wearing away of bedrock by continuous friction caused
by sand or rock fragments in water, wind, and ice.

Atmospheric pressure:

The pressure exerted by the weight of air over a given area of
Earth's surface.

Bajada:

Several alluvial fans that have joined together.

Basin:

A hollow or depression in Earth's surface with no outlet for
water.

Compression:

The reduction in the mass or volume of something by applying pressure.

Crest:

The highest point or level; summit.

Deflation:

The lowering of the land surface due to the removal of fine-grained
particles by the wind.

Ecosystem:

A system formed by the interaction of a community of plants, animals,
and microorganisms with their environment.

Eolian:

Formed or deposited by the action of the wind.

Erg:

A vast area deeply covered with sand and topped with dunes.

Erosion:

The gradual wearing away of Earth surfaces through the action of wind
and water.

Gully:

A channel cut into Earth's surface by running water, especially
after a heavy rain.

Leeward:

On or toward the side facing away from the wind.

Saltation:

The jumping movement of sand caused by the wind.

Silt:

Fine earthy particles smaller than sand carried by moving water and
deposited as a sediment.

Slip face:

The steeply sloped side of a dune that faces away from the wind.

Surface creep:

The rolling and pushing of sand and slightly larger particles by the
wind.

Ventifact:

A stone or bedrock surface that has been shaped or eroded by the wind.

Windward:

On or toward the side facing into the wind.

A star dune forms where there is plentiful sand and many dominant winds
come from various directions. As its name implies, a star dune resembles a
star with its many arms pointing out in different directions. The crests
on the arms slope upward, meeting to form a point in the middle

Four of the most common dune forms: barchan, transverse,
longitudinal (or linear), and blowout (or parabolic).

of the dune similar to that of a pyramid. The largest and highest dunes
are star dunes.

All the five major dunes can be further categorized into simple, compound,
and complex types. When they occur in their original states, all
dunes are simple. When a smaller dune forms on top of a larger dune of a
similar type and orientation to the wind, the entire structure is known as
a compound dune. When a smaller dune forms on top of a larger dune of a
different type, it is known as a complex dune.

Alluvial fan

Water may be scarce in a desert environment, but when it does appear, such
as in a brief and violent rainstorm, it can change the landscape quickly
and dramatically. Precipitation that falls in higher elevations in deserts
flows rapidly down to flat areas through canyons, valleys, and other
narrow, confined channels. Because most desert soil lacks plants and their
root systems to help hold the soil together, the flowing water easily
picks up any loose material in its path. The faster the water flows, the
larger the pieces of material it is able to pick up and carry along.

When the rushing water finally reaches a plain or flat area, it loses
power since gravity is no longer helping it flow down a steep slope. As it
slows, the water is unable to carry the sediment—gravel, clay,
sand, and silt—it picked up on its way downhill. Large rocks and
other heavy material are deposited first at the base of the canyon,
followed by other material in decreasing size. No longer confined to a
narrow channel, the water spreads out the farther it moves away from the
base of the canyon. The finest material it carried is deposited at the
outer edge. When the water evaporates, the sediments remain behind. Over
time, as more water flows onto the plain, more sediment is deposited, and
a wide, fan-shaped pile known as an alluvial (pronounced ah-LOO-vee-al)
fan forms. When two or more alluvial fans merge on a plain to create a
broad, sloping surface, they form a bajada (pronounced ba-HA-da; Spanish
for slope).

Arroyo

An arroyo (pronounced ah-ROY-oh) is another desert landform sculpted by
the action of water. Sudden heavy downpours cut channels in the desert
floor, often in canyons or other low-lying areas. These fast-moving but
short-lived streams create deeper channels or gullies with steep sides and
an almost flat bed or bottom. Just as quickly as the water appears, it
disappears in the normally dry desert environment. What remains is an
arroyo, a dry watercourse with a floor that is often gravel-strewn.

Blowout

Following water, wind is a major cause of erosion (the gradual wearing
away of Earth's surfaces through the action of wind and water) in
the desert. Without plants and their anchoring roots, loose desert soil is
moved easily by near-constant blowing winds. Blowouts, also known as
deflation basins or hollows, are depressions made in sand or light soil by

Furnace Greek Formation, Death Valley National Park, California,
which consists primarly of alluvial fan and playa deposits.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

strong wind action. These low spots may range in size from several feet to
several miles in diameter. Blowouts can form around desert plants with
hardy roots or around rock structures, leaving them perched atop a column
as more and more sand or soil is blown away.

Desert pavement

The strength of the wind determines the amount and type of material it
removes from the desert floor. As the wind increases in strength, it is
able to move and transport more and larger particles. Initially, very fine
particles are moved by the wind. As more and more of these types of
particles are removed, the surface of the land lowers in elevation. This
action, known as deflation, continues until what remains on the desert
floor is a layer of closely packed pebbles and rocks too heavy for the
wind to move. Settled and wind-polished, the entire surface is called
desert pavement or reg. The older the pavement, the smoother and flatter
it appears, like a highly worn cobblestone street. Younger pavement areas
are coarser, less eroded by the action of the wind.

Oasis

Beneath Earth's surface, water fills the pore spaces and openings
between rocks. This water that seeped through the soil, drawn downward by
gravity, is known as groundwater. At a certain level below ground, all
openings between the rocks are completely filled with groundwater. The
upper surface of this area is called the water table, and it is found
everywhere beneath Earth's surface. Even beneath desert regions,
there is water.

Groundwater hardly ever reaches the desert surface, but when it does, it
can transform the stark landscape into a fertile haven thriving with many
species of plants and animals that otherwise would not exist in that hot,
dry space. This green area, existing like an island in a sand sea, is an
oasis (pronounced oh-AY-sis; plural form is oases). Many oases are
artificial ecosystems, created by people living in the desert using large
pipes to tap into the groundwater to bring it to the surface. A few,
though, are the result of natural forces. These oases are centered on
springs that have been exposed because of blowouts and other erosive
actions by the wind that have lowered the land surface.

Playa

Permanent water bodies are rare in deserts. When precipitation does occur
in a desert, it often runs down steep hills to form temporary surging
streams in low-lying areas before evaporating or sinking into the ground.
When the water falls on fairly flat areas, it may collect in a basin or
other slightly depressed area, forming a small lake that may last for a
while before the water evaporates or is absorbed. What remains after the
water
is gone are the sediments it collected as it flowed along the desert
surface. Those sediments, mostly clay, silt, and various salts, form a
level, broad, cracked surface called a playa (pronounced PLY-uh; Spanish
for beach). When water is still present, these bodies are called playa
lakes.

Although they are very rare, permanent desert lakes do exist. Two examples
are the Great Salt Lake of Utah and the Dead Sea of Israel and Jordan. The
Great Salt Lake is all that remains of ancient Lake Bonneville, which was
ten times the size of the Great Salt Lake, while the Dead Sea was once
part of the Mediterranean Sea.

Yardang

Among the most striking desert landforms created by the action of the wind
are yardangs (from the Turkic word
yar,
meaning "steep slope"). A yardang is a wind-sculpted,
streamlined ridge that can stretch for over one mile in length and 100
feet (30 meters) in height. It forms when strong winds blowing primarily
in one direction remove all sand in an area down to the bedrock (solid
rock beneath the sand). If the bedrock is slightly soft or porous, winds
will erode the bedrock, sandblasting hollows out of the soft parts of its
surface. Over a vast amount of time, winds cut away enough material to
leave a sleek-shaped ridge, similar in shape to the bottom of an
overturned boat, that runs parallel to the wind.

Forces and changes: Construction and destruction

The desert landscape is shaped primarily by two forces: wind and water.
Through three main actions, wind sculpts the face of the desert landscape.
It is the more prevalent force, but water is the more powerful. Though its
appearance is limited, water is the primary agent of erosion in the
desert.

To understand the actions of wind and water in forming desert land-forms,
it is necessary to understand first how circulating air patterns in
Earth's atmosphere create conditions that bring about desert
environments. As mentioned earlier, the majority of the planet's
deserts are located in two horizontal belts near the equator, where the
Sun is closest to Earth. In this region, the Sun heats the air, causing it
to rise. As the air rises, low air pressure develops beneath it. As the
air rises even higher in the atmosphere, it cools, and moisture in the
cooling air begins to condense and fall as rain (this explains why most
rainforests are also found near the equator). When the cool air reaches
the top of the troposphere, the lowest 10 miles (16 kilometers) of
Earth's atmosphere, it can rise no further and begins to move
toward the poles, cooling even more as it travels northward and southward.

At about 30° latitude north and south of the equator, the cool, dry
air descends to Earth's surface. In the process, the air becomes
strongly heated

Was the Sphinx Originally a Landform?

In 2001 Farouk El-Baz, Boston University professor and director of the
university's Center for Remote Sensing, published a paper in
which he suggested that the pyramids and the Great Sphinx located on
Egypt's Giza Plateau were based on natural landforms found in the
eastern Sahara Desert. El-Baz pointed out that the landscape of the Nile
River valley features coneshaped hills that have lasted many years
because their shape forces the strong winds in the area upward,
preventing the wind from eroding them or wearing them down. The pyramid
builders, El-Baz believes, would have looked to these landforms in their
quest to build lasting structures.

Extending his theory even further, El-Baz asserted that the Great
Sphinx, the enormous sculpture with the head of a man and the body of a
reclining lion, might even have been carved by ancient Egyptians in 2500
B.C.E.
from an existing desert landform. He cited the works of early
twentieth-century explorers and geologists that described wind-eroded
yardangs in northwestern China and southwestern Egypt as
"sphinx-like" or "lionlike." El-Baz believes
the head of the sphinx was an existing yardang (a wind-sculpted ridge)
the Egyptians reshaped. They then formed the body, which sits in a
hollow or depression, by digging out the naturally occurring limestone
on the plateau around it.

Great Sphinx, Egypt.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

due to compression caused by atmospheric pressure (atmospheric pressure
increases closer to the planet's surface). During its descent, the
warming air pushes the air below it back toward the equator, since air
flows always move toward areas of low pressure. Passing over land on its
way back to the equator, the now heated, dry air evaporates any moisture
in the air, creating dry regions or deserts.

These wind patterns explain the formation of many of the world's
deserts in the area between 15° and 35° latitude north and
south of the equator. Although much weaker, similar atmospheric
circulation (with heated, dry air evaporating moisture as it moves over
Earth's surface) occurs over both poles, creating polar deserts.
While heated because of compression by the atmosphere, the air over the
polar regions is not as warm as in equatorial regions simply because the
Sun is farther away from Earth in these areas. Polar deserts, however, are
similar to hot deserts because they have very low humidity and
precipitation levels.

The topography (physical features) of Earth in combination with
atmospheric pressure creates other deserts around the planet. Rain-shadow
deserts are those that lie on the leeward side of mountain ranges located
near coasts. As moisture-laden air flows inland from an ocean and
encounters a mountain range, it is pushed upward. Cooling as it rises, the
air begins to lose its moisture on the windward side of the mountain range
through rainfall. Once on the leeward side, in the "shadow"
of the mountain range, the air has little moisture left. Heated by
compression as it descends, the warm, dry air forms deserts in the slope
of the range. All deserts in North America are formed by this action.

The Literary Landscape

"What land can equal the desert with its wide plains, its grim
mountains, and its expanding canopy of sky! You shall never see
elsewhere as here the dome, the pinnacle, the minaret fretted with
golden fire at sunrise and sunset; you shall never see elsewhere as here
the sunset valleys swimming in a pink and lilac haze, the great mesas
and plateaus fading into blue distance, the gorges and canyons banked
full of purple shadow. Never again shall you see such light and air and
color."

—John C. Van Dyke,
The Desert,
1901.

The force of wind

Wind is a common element in desert environments because the Sun heats air
near the desert surface, causing it to rise. The warmed air is then
replaced by cooler air, which is then heated and rises. This constant
cycle of air warming, rising, and being replaced results in winds. The
lack of desert vegetation also allows winds to blow unrestricted. Strong
and unchecked, wind has the ability to transport, erode, and deposit
material in the desert, creating and modifying its landforms. The work of
the wind

The circulating air patterns in Earth's atmosphere.

is known as eolian (pronounced ee-OH-lee-an; from Aeolus, the god of the
winds in Greek mythology).

The wind moves like a fluid, like water, and it can erode only if it is
strong enough. Very often, it merely transports material. Very small
particles, measuring less than 0.01 inch (0.02 centimeter) in diameter,
can be picked up easily by desert winds and carried aloft for hundreds or
thousands of miles. Suspended on air currents, whirling high in the
atmosphere, dust from Africa's Sahara Desert sometimes crosses the
Atlantic Ocean before landing in the west Atlantic and Caribbean Sea. On
the other hand, sand particles, which typically measure 0.01 to 0.25 inch
(0.02 to 0.64 centimeter) in diameter, can be carried only by extremely
strong winds. Silt and other very small-sized particles fill the air
during dust storms, but these and most other wind-borne grains are too
small to cause erosion or sandblasting of major landforms that stand high
above the desert floor.

Scientific experiments have shown that wind-blown sediment causes the most
erosion at a height of no more than 10 inches (25 centimeters). Indeed,
erosion of the landscape by the wind takes place mainly on the
ground, where the wind removes fine-grained particles causing deflation
(lowering of the land surface due to the removal of particles by the
wind). This continued action leads to blowouts and desert pavement. Any
stone or part of the bedrock that has been abraded or shaped by the wind
is known as a ventifact (artifact of the wind). A yardang is one large
desert landform that is sculpted by the wind though deflation and
abrasion.

The wind transports larger-grained sediments, particularly sand, through
the process called saltation. While light enough to be picked up by strong
wind, sand is too heavy to remain suspended in the air. As a result, it is
moved along Earth's surface by the wind through a series of short
jumps and bounces. The vast majority of sand transported in this way
travels within 2 feet (0.6 meter) of the ground. As saltating particles
crash to the ground, they can dislodge and move slightly larger particles,
such as small pebbles. The sliding and rolling movement of these particles
is called surface creep.

Cultural Landforms

In the barren Great Sandy Desert in Australia's Northern
Territory a red sandstone monolith (tall block of rock), the
world's largest, rises 1,143 feet (348 meters) above the
surrounding flat sandy plain. Ayers Rock, known as Uluru to the Anangu
Aboriginals, the native people of the region, is an inselberg, a
prominent steep-sided hill composed of strong, solid rock that sits
isolated in a desert plain. Like other inselbergs, Ayers Rock's
rounded appearance is caused by weathering in which its durable surface
has been eroded in successive layers, like the peeling of an onion skin.

Estimated to be between 400 and 600 million years old, the monolith
measures about 1.5 miles (2.4 kilometers) wide and 2.2 miles (3.5
kilometers) in length. Geologists believe that two-thirds of the rock,
measuring some 3.7 miles (6 kilometers), lies beneath the desert floor.
The rock mound's changing colors as the Sun moves over it, from
gray to gold to red to purple, are due to minerals like feldspar
contained in the sandstone. The famous red color is also a direct result
of iron in the sandstone, which oxidizes or rusts.

Uluru has deep cultural significance for the Anangu. They consider it a
sacred place, having been formed by supernatural beings who crossed the
Australian landscape, creating various landforms in their wake. In 1985,
the Australian government returned Uluru National Park, which contains
the monolith, to its original owners. The Aboriginals then leased the
area to the Australian National Parks and Wildlife Service.

Sand is finally deposited when the wind encounters some type of an
obstacle: rocks, vegetation, or a manmade structure. As wind passes over
an obstacle, the wind's velocity, or speed, increases. Once on the
other

Ayers Rock, Australia.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

side of the obstacle, its velocity decreases and any sand or particles it
was transporting drop out to begin forming a mound. When there is a steady
supply of sand carried by a steady wind that comes in contact with an
obstacle or irregularity in the flat ground surface, a sand dune forms.

The type of dune formed is influenced by the direction and strength of the
wind, the amount of sand it carries, and the shape of the land. All dunes
have a gently sloping windward face and a steeply sloping leeward or slip
face. The slope of the windward face is usually between 10° and
20°, while the slip face has a slope of a much greater angle, about
32°. The windward face is usually hard packed and smooth, cut
occasionally by minor grooves. The slip face is soft and unstable.

As wind passes over the windward face of a dune, it moves sand along the
surface through sliding movements and saltation (the jumping movement of
sand caused by the wind). Once over the crest of the dune, sand flows down
the slip face. This action—the eroding of sand on the windward face
and the deposition of sand on the slip face—causes the dune to move
or migrate with the wind like a slow wave.

It is rare that a single dune forms in an area. Most often, dunes form in
groups called fields on broad flat lands where winds blow steadily and
sand is plentiful. Far-reaching fields, such as those in the Arabian
Desert in eastern Egypt, are called sand seas or ergs (Arabic for ocean).

The force of water

Deserts are defined by their lack of water, both rainfall and
free-standing. Coastal deserts may experience one or two rainfalls a year;
those farther inland may receive one or two a decade. Though rare, desert
rainfalls are often heavy storms lasting mere minutes or a few hours. The
Sahara Desert, which normally receives less than 5 inches (12.7
centimeters) of rain a year, once received 1.7 inches (4.4 centimeters) in
three hours.

Water is a natural force of erosion everywhere on the planet, but
especially so in the desert. When raindrops strike bare ground that is not
protected by vegetation, they loosen particles of soil, spattering them in
all directions. During heavy rains on sloped surfaces, the dislodged soil
is carried off in a flow of water.

With little vegetation or organic-rich soil to absorb the water, the
desert landscape is quickly eroded as the torrent of water surges over it,
picking up and transporting as much sediment as it can carry. What takes
desert wind a year to accomplish, water surpasses in a day or two. Because
of the unexpected, and often violent, nature of rainfall in the desert,
more people die of drowning than of thirst in that arid environment.

Aided by gravity and steep slopes, rainfall on high desert elevations
flows down over the surface as sheets, picking up sediments along the way.
Finding natural depressions, such as gullies and canyons, water continues
to race along, gaining speed and power as it is confined and flows
downward. Increased velocity allows the water to pick up more and larger
sediments and rock debris, eroding them and the surface below as it rushes
along. Often clogged with so much debris, the water can resemble a
mud-flow (a thick mixture of water, mud, and other surface fragments).

Arroyos, dry streambeds created by previous rains, again fill with water.
When an arroyo finally opens onto a flat, broad plain, the rushing water
flows out and drops its load of sediments, forming a new alluvial fan. In
other areas, basins or depressions in the desert floor fill with water,
forming playa lakes that soon evaporate, leaving a dry, cracked, salty
lake bed that will remain until the next rain.

Spotlight on famous forms

Algodones Sand Dunes, California

The Algodones Sand Dunes, also known as the Imperial Sand Dunes, stretch
more than 40 miles (64 kilometers) northward from the U.S.-Mexico

Rippled sand dunes of the Algodones Sand Dunes, California. The dune
system, one of the largest in North America, covers an area of about
1,000 square miles in the southeastern portion of the state of
California.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

border along the eastern edge of the Imperial Valley agricultural region
in southeastern California. The dune system, the largest in the state and
one of the largest in the country, formed approximately ten thousand to
twenty thousand years ago. Some geologists believe that blowing sands from
Lake Cahuilla (pronounced kah-WEE-ah), an ancient freshwater lake located
just west of the present-day area, helped create the dunes.

The dune system, which lies within the Sonoran Desert, varies in width
from 5 to 8 miles (8 to 12.9 kilometers). Prevailing westerly winds cause
the dunes to migrate to the southeast at a rate of approximately 1 foot
(0.3 meter) a year. The crests of the largest dunes often reach 300 feet
(91 meters) above the surrounding landscape. With summer temperatures
rising above 110°F (43°C) and annual rainfalls of less than
2 inches (5 centimeters), the system supports very little vegetation.
Often, the dunes stretch uninterrupted by any life for miles, a vast pile
of pure golden sand against a dry blue sky. This unspoiled environment has
been used as the set for many films, including
Star Wars
(1977) and
Return of the Jedi
(1983).

This ancient and fragile dune system is threatened by uncontrolled and
intense use of off-road vehicles. The U.S. Bureau of Land Management has
set aside over 70,000 acres (28,000 hectares) of the dune system as a
protected environment free from such recreational activity.

Lut Desert yardangs, Iran

The Lut Desert or Dasht-e Lut ("Desert of Emptiness") lies
in Iran's southeastern province of Kerman. The great sand and stone
desert covers an area almost 300 miles (480 kilometers) in length and
almost 200 miles (320 kilometers) in width. Among the driest places on the
planet, it receives an average of only 1.2 inches (3 centimeters) of rain
a year. Certain areas of the desert reportedly receive no rain. Extremely
barren, the desert contains the only region free from any life, including
the existence of bacteria, on Earth.

In the western part of this desolate environment lie some of the
world's most prominent yardangs. Rising up to 282 feet (80 meters),
these streamlined ridges have been carved by the wind out of the silty
clay and sand lining the desert floor. Lying parallel to the prevailing
north-north-west winds, the yardangs are separated by troughs measuring
330 feet (100 meters) or more. The crests or summits of the largest of
these yardangs are rounded or flat; all others are narrow. Geologists have
found no evidence that water has played any significant part in their
creation and continued erosion.

Racetrack Playa, Death Valley, California

Death Valley, located in eastern California, lies between the Panamint
Mountains on the west and the Amargosa Range on the east. A desert basin,
much of which lies below sea level, Death Valley contains a small pool,
Badwater, that is the lowest point in the Western Hemisphere: 282 feet (86
meters) below sea level. Death Valley is also the hottest place on the
North American continent, with summer temperatures often exceeding
120°F (49°C). Rainfall seldom measures more than 2 inches (5
centimeters) a year.

Located within Death Valley National Park is a playa known as Racetrack
Playa. Like other dry lakebeds in other deserts around the world,
Racetrack Playa temporarily fills with water during heavy storms,
collecting the runoff from nearby mountain slopes. Under the hot Sun, the
thin sheet of water quickly evaporates, leaving the almost perfectly flat
clay surface of the playa dry and cracked in a mosaic pattern.

What makes Racetrack Playa unique, and world-famous, are the rocks that
slide across its surface and the mystery behind their movements.
Periodically, rocks from rock formations around the playa break off and
drop

The mysteriously "sliding" rocks of Racetrack Playa in
Death Valley, California. Researchers believe the movement is the
effect of strong winds at a time when the normally dry lake bed
becomes muddy from rain.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

to the floor of the dry lakebed. Some of these rocks are actually
boulders, weighing up to 705 pounds (320 kilograms). Instead of being
concentrated in one area, these rocks are scattered across the playa, with
grooved trails stretching behind them, some straight, some curved, some
extending as long as 2,896 feet (880 meters). Until recently, geologists
have been at a loss to explain the possible reason behind their movement.

Now, geologists believe the rocks slide because of a combination of water
and wind. After the playa becomes wet with rainwater, and before the water
completely evaporates, the surface of the clay-filled lakebed is extremely
slick. Winds, channeled by low points in the surrounding mountains, stream
across the playa in natural wind tunnels. Some of the winds reach 70 miles
(113 kilometers) per hour. Geologists believe the winds are strong enough
to set even the heaviest boulders sliding across the slippery playa
surface.

Sahara Desert dunes, North Africa

The Sahara Desert, the world's largest, spreads across the upper
third of the African continent from the Atlantic Ocean to the Red Sea.
From north to south, it extends about 1,200 miles (1,930 kilometers). In
all, it
covers about 3.5 million square miles (8.8 million square kilometers), an
area almost as large as the United States. Countries that fall at least
partly within the Sahara include Morocco, Algeria, Tunisia, Libya, Egypt,
Mauritania, Mali, Niger, Chad, and Sudan.

The desert's landforms, which tend to have a golden color, range
from rocky mountains and wind-eroded highlands (known as hammada) to
rock-filled plains (known as reg) and deep, narrow canyons. Oases dot the
landscape, providing native people, plants, and animals the chance to
exist.

Shuttles on the Playa

Edwards Air Force Base, home of the U.S. Air Force Test Center, is
located in the Mojave Desert about 100 miles (161 kilometers) northeast
of Los Angeles, California. Adjacent to the main Edwards complex lies a
44-square-mile (114-square-kilometer) playa known as Rogers Dry Lake.
For more than forty years, the dry lakebed has been used for emergency
and test landings of experimental and standard aircraft. Since 1977, the
playa has served as the landing site for many space shuttle tests and
operational flights.

Formed some 2.5 million years ago during the Pleistocene Epoch, Rogers
Dry Lake is the largest geological formation of its kind in the world.
Its extremely flat, hard surface of parched clay and silt undergoes a
cycle of renewal each year. Desert winds sweep water from winter rains
back and forth across its surface, smoothing it out to an almost glassy
flatness. Its surface is so perfect, in fact, that runways can merely be
painted on the ground. There are seven outlined runways on the playa,
the longest of which extends 7.5 miles (12 kilometers).

Many aviation milestones have been reached on the playa. In 1947, U.S.
Air Force Captain Chuck Yeager became the first person to break the
sound barrier when he piloted the rocket-powered Bell X-1 over the playa
to a speed of Mach 1.06, exceeding 662 miles (1,065 kilometers) per hour
at 40,000 feet (12,192 meters) elevation.

Just thirty years later, the U.S. space shuttle
Enterprise
successfully completed a landing and roll out on Rogers Dry Lake after
it had been launched from the back of a 747 aircraft. This, along with
four subsequent tests, demonstrated the soundness of the shuttle design.
In the early days of the space shuttle program, most shuttles landed on
the playa. Present-day shuttles continue to land there if weather
conditions are poor at the Kennedy Space Center in Cape Canaveral,
Florida, home to the space shuttle program.

Overshadowing all these distinct landforms, however, are the great Saharan
sand seas or ergs, which compose 15 percent of the desert. Confined to
large basins, the ergs are separated by plateaus and low mountain ridges.
Dunes in the ergs can soar to 1,000 feet (305 meters). While some dunes in

Space shuttle on the back of a 747 at Rogers Dry Lake, California.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

the Sahara are stationary, others are in constant motion. As they migrate
with the wind, dunes may emit strange singing or booming sounds as the
sand grains move against each other, tumbling down the slip faces of the
dunes.

It is not uncommon for dune fields in the Sahara to stretch for hundreds
of miles without a break or any sign of life. The Libyan Erg, located near
Egypt, holds the greatest mass of dunes on Earth. It covers more than
200,000 square miles (518,000 square kilometers), an area almost as large
as France.

White Sands National Monument, New Mexico

Of North America's four deserts—the Chihuahuan, the Great
Basin, the Mojave, and the Sonoran—the Chihuahuan is the farthest
east and the farthest south. Extending north from Mexico, the desert
reaches into

Beni Abbes Dunes in the Sahara Desert, Africa. The Sahara Desert is
the world's largest desert, covering about 3.5 million square
miles—an area almost as large as the United States.
PHOTOGRAPH REPRODUCED BY PERMISSION OF

PHOTO RESEARCHERS, INC.

southern New Mexico, southeastern Arizona, and southwestern Texas. At the
desert's northern end in New Mexico lies a mountain-ringed valley
called the Tularosa Basin. Covering 275 square miles (712 square
kilometers) of this basin is a dune field unlike any other in the world.

The glistening white dunes in this basin form the largest pure gypsum dune
field in the world. Most sand on the planet is made of quartz, one of the
most abundant rock-forming minerals found in Earth's crust. Gypsum
is a mineral composed of calcium sulfate (calcium, sulfur, and oxygen) and
water. It rarely forms sand because it is soluble (can be easily
dissolved) in water.

Precipitation that falls on the mountains surrounding Tularosa Basin,
averaging 8 inches (20 centimeters) a year, dissolves gypsum contained in
the rocks. The flowing water then carries the gypsum and other sediments
down into the basin into a playa, Lake Lucero, which lies a few miles
southwest of the dune field. With no watercourse, such as a river, to
carry the water away, it evaporates, leaving the now-crystallized gypsum
on the surface of the dry lakebed. Strong southwest winds blowing across
the playa transport the gypsum to the nearby dune field. Quite dynamic,
many dunes in the field migrate with the wind, moving northeast as much as
30 feet (9 meters) a year.

White Sands National Monument, in the northern end of the Chihuahuan
Desert, New Mexico. Here, wavelike dunes of gypsum sand have
engulfed 275 square miles of the desert, creating the largest gypsum
dune field in the world.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

CORBIS CORPORATION

.

A large portion of the dune field, 115 square miles (298 square
kilometers), is enclosed in the White Sands National Monument, which helps
preserve the ecological integrity of the region. Surrounding the park is
the White Sands Missile Range, a military base used for testing various
weapons. Within the base lies Trinity Site, an area where the first atomic
bomb was detonated on July 16, 1945.

For More Information

Books

Gallant, Roy A.
Sand on the Move: The Story of Dunes.
New York: Franklin Watts, 1997.

The webpages on desert studies is one of the one most comprehensive that is availabe. As a student in Hydrology/Climatology in my country, it is hard to get hard copy of such information.
I thank you very much.

"the air over the polar regions is not as warm as in equatorial regions simply because the Sun is farther away from Earth in these areas." This should be corrected. The distance of the sun from the earth in these areas does not matter. It is the angle the sunlight hitting the area. Around the equator, the sunlight hits the earth head-on (90 degrees), as you move up the latitudes, there is less sunlight per surface area due to the angle. At the poles the angles will be around 0 degrees.

As a geography teacher this article has been of very great contribution in my teaching.It has surely broadened my understanding on various features that i never knew them before.but the only problem is on feature by the name of sebkha/playa is some sort of confusion because in my understanding I thought it was a"temporal fluctuating salt lake in arid and semi-arid regions"

Also life can exist in desert areas for both living things and plants,as human beings can survival in desert through nomadic activities and engage in agriculture activities,those activities can be well conducted in semi arid areas, animals like camel,snakes are found in desert while in semi arid areas animals like carnivores and herbivorous.
plants which have long roots to tap deep moisture from underground such as cuct and other spine plants